Noise reduction system



April 23, 1940.

v. D. LANDON 2,197,937 NOISE REDUCTION SYSTEM Filed May 27, 1958 2 Sheets-Sheet 1 Cay (7 63471967 7' Y FROM (#1955/5 P0455 7'0 COUNTEAPOASE E 1V0 OFPP/Mfl/PY WIND/N6 Zmm April 23, 1940. v. D. LANDON 2,197,937

NO ISE REDUCTION SYSTEM Filed May 27, 1938 2 Sheets-Sheet 2 lnnentor Patented Apr. 23, 1940 UNITED STATES NOISE REDUCTION SYSTEM Vernon D.,Landon, Haddonfield, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application May 27, 1938, Serial No. 210,323 6 Claims. (01. 250-20) 1 This invention relates to noise reduction systems such as are useful in eliminating the undesired noise produced in radio. receiving apparatus or the like by electrical impulsesother than 5 those of the desired signal, and has for its principal object the provisionof an improved system and method of operation for eliminating the noise resulting from extraneous impulses such as those which are transmitted over the power supply line and tend to produce a noise voltage between the apparatus chassis and ground.

It has been found that much of the noise encountered in radio reception is due to undesired electrical impulses which come into the receiver over the power supply line and produce a noise voltage between the receiver chassis and ground. While this noise voltage may be reduced by the use of a short ground lead, the use of a sufiiciently short lead is usually impracticable. In

90 accordance with one modification of the invention, this difiiculty is avoided by the provision of an improved noise reduction system including long and short antennae which are interconnected through the primary winding of a trans-v former provided with a tuned secondary winding through which signal impulses are applied to the input circuit of the first stage of the amplifier. These two antennae may be spaced from one another by a distance of a few inches to a few feet. An additional feature of this modification of the invention is the provision of a variable balancing condenser connected between the long antenna and the chassis. f

If no capacity coupling existed between the primary winding andthe secondary winding or chassis, this balancing condenser would not be required. Due to the existence of such capacity, however, the noise voltage on the chassis induces noise voltage in both the antennae. The function of the balancing condenser is to neutralize the noise voltage otherwise produced in the secondary winding and in the amplifier input circuit.

If the noise voltages induced in the two antennae were equal in phase and amplitude, no

noise current would follow in the primary winding. The long antenna, however, is generally of lower impedance and had less noise voltage induced in it until the condenser is adjusted to produce abalance.

If capacity exists between the primary winding and the high potential side of. the secondary winding, a change in thebalance point is desirable for the reason that neutralization of this capacity minimizes the effect of frequency change on the balance point. 7 V 1 I A second'modification of theinvention is directly applicable to the conventional antenna and receiver and is distinguished from that previously described in that the balance is not deleteriously affected by capacity coupling between 5 the primary and secondary windings of the antenna transformer. a

- This second'modification involvesthe addition of four units to the conventional'receiver. Thus a series condenser is interposed in the ground -10 lead, (2) a bifilar wound choke coil is connected in series with the powersupply leads, (3) a condenser is connected in shunt to the choke coil, and (4) a condenser is interposed between the antenna and the power side of Y the choke coil.

Any one of these condensers may be made adiustable for balancing out the noise and a'resistor may be connected in series with one of the condensers for improving the balance. If a portion of the noise is picked up on the antenna or .2

ground lead, the only effect is to change the balance point.

A third modification of the invention involves (l) the provision of a bifilar choke coil in the power supply leads as previously described, (2) 2.25 the use of a small inductance in the ground lead of the chassis, (3) the connection of the balancing condenser between the antenna and the power side oflthe choke coil, and (4) theprovision 'of. a" variable resistor either in parallel or 30 series with the balancing condenser. With this modification, a very low noise level is obtained when the resistorand condenser are adjusted for minimum noise, and the adjustment required does not vary with frequency if the'bifilar choke m5 coil resonates at the same frequency as the antenna. I f

' The invention will be better understood from the following description'considered in connection with the accompanying drawings and its 440 scope is'indicated by the appended claims.

Referring to the drawings,

Figure 1 is an explanatory diagram illustratinga circuit equivalent to the circuits of Figs. 2,, 3 and 4,' 45 Figure 2 is a wiring diagram of a noise reduction system involving the invention,

Figures 3 and 4 are similar diagrams illustrating modified forms 'of the system of Fig. 2',

Figure 5 is a wiring diagram of a modified 450 .form of the invention,

The system of Fig. 2 includes a long antenna l0 and a short antenna l I between which is connected the primary winding H. of a coupling transformer. The secondary winding l3 of this transformer is tuned by a condenser l4 and connected between the cathode and control grid of the first amplifier stage l5. It will be noted that the cathode is connected to the receiver chassis, as indicated at l6, and that-an adjustable balancing condenser is connected between the chassis and long antenna In for superimposing on this antenna sufficient additional noise voltage to balance that induced in the short antenna ll through the capacity coupling between the chassis and ground.

With these connections, the balancing condenser I'I may be so adjusted that the noise voltages of the two antennae are equal and no noise voltage appears in the primary winding l2. The primary winding l2 should, of course, resonate at a frequency outsidethe reception band of frequencies. A simplified circuit equivalent to that of Fig. 2 is shown by Fig. 1 wherein the noise voltage source (in this case the power supply leads) is indicated at l8.

Fig. 3 shows a variable resistor l9 connected in series with the balancing condenser l1. Such a resistor makes possible a. much lower noise minimum. This resistor is equally effective when connected in shunt to the balancing condenser, and may sometimes be connected. between the chassis and the short antenna. The use of the resistor l9 corresponds to the phase adjustment of an alternating current bridge and permits almost perfect elimination of the noise originating at a single source. The balance adjustment may be somewhat different for diflerent sources of noise.

The short antenna H may be replaced by a ground lead with a small series condenser 20, as illustrated by Fig. 4. If the capacity of the condenser 20 is smaller than that of the antenna ill, the balancing condenser must be connected to the antenna. If the capacity of the condenser 28 is larger than that of the antenna, the balancing condenser must be connected to the ground lead. In cases where there is sufficient capacity coupling between the chassis and primary winding, the balancing condenser and resistor may be omitted and the'condenser 20 adjusted to establish the balance. It is, furthermore, feasible to connect the variable resistance in series with the antenna or ground lead without detracting from the operation of the system.

Fig. 5 illustrates a noise reduction system which is applicable to any conventional type of antenna and radio receiver. This system includes a bifilar choke coil 21! connected in the power supply leads, a condenser 22 connected in shunt to the choke coil 2|, a condenser 23 connected between the antenna l0 and the power end of the coil 2|, and a condenser 24 connected in the ground lead 25. A bypass condenser 25 may be connected between the power supply leads, as indicated, if found to be desirable. While the condenser 22 has been shown as adjustable for the purpose of balancing out the noise, it should be understood that the same result may be achieved by adjustment of the condenser 23 or the condenser 24. A variable resistor connected in series with one of the condensers may be utilized as explained in connection with the systems of Figs. 3 and 4.

The system of Fig. 5 will work on any type of receiver. In this case, capacity coupling between the primary and secondary winding does no harm and, if noise is picked up on the antenna or ground lead, it only changes the balance point.

The adjustment of the system of Fig. 5 is similar to that of balancing a capacity bridge. Considering the source of noise to be the power sup ply leads, it is seen that there is a capacity voltage divider applying the noise voltage to the antenna and another applying a noise voltage to the chassis. If the noise voltage applied to the antenna is equal to that applied to the chassis, there is no noise input to the receiver. If the resistance of the ground is greater than that of the antenna, a variable resistor may be interposed at one of the two points marked X to improve the balance. If the antenna has the higher resistance, the variable resistor should be inserted at one of the points marked Y. Since the antenna is capacitive, the balance will remain about the same over the broadcast range. The eifective antenna capacity will increase slightly at high frequencies due to its inductance, and a small inductance may be inserted at one of the points Y to compensate for this.

The system of Fig. 6 includes (1) the bifilar choke coil 2| connected in the power supply leads, (2) a small inductance 21 connected in the ground lead of the chassis, (3) a balancing condenser 28 connected between the antenna and the power end of the choke coil 2|.- A variable resistor 29 is provided either in series or in parallel with the balancing condenser 28. A condenser 30, connected in shunt to the bifilar choke coil 2 i, may be utilized to maintain the balance at all frequencies. Under certain conditions, the distributed capacity of the choke coil 2| may be utilized for the same purpose.

The inductance 21 may be small if a good are ternal ground is provided. Balancing is then less critical. ground lead, the balance point may be off the scale of the balancing adjustment unless the inductance of the inductor 21 is increased.

Analytically, the circuit of Fig. 6 is as illustrated by Fig. 7 which is for the most part selfexplanatory in view of the information printed opposite its various elements. The. use of the variable resistor in connection with the balancing condenser is made necessary by the high resistance of the ground. It is seen that where the antenna is series resonant the choke coil is parallel resonant and the bridge balance is maintained, the antenna post A and ground (3- being of the same potential.

I claim as my invention:

1. In a radio apparatus including an input transformer primary winding provided with a pair of terminals, an antenna connected to one of said terminals and a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of a circuit connected to the other of said primary terminals, and means including a capacitor and a resistor connected in series with said capacitor for applying to one of said terminals a component of said noise voltage whereby said terminals are maintained at the same potential with respect to said voltage.

2. In a radio apparatus including an input transformer primary winding provided with a pair of terminals, an antenna connected to one of said terminals and a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of a circuit connected to the other of said primary terminals, and means including a capacitor and a resistor connected in If some noise voltage is induced in the series between said chassis and said antenna for applying to said antenna a component of said noise voltage wherebyf said terminals are maintained at the same potential with respect to said voltage.

3. In a radio apparatus including an input transformer primary Winding provided with a pair of terminals, an antenna connected to one of said terminals and a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of a relatively short antenna connected to the other of said terminals, and means for applying to one of said terminals a component of said noise voltage whereby said terminals are maintained at the same potential with respect to said voltage.

4. In a radio apparatus including an input transformer primary winding provided with a pair of terminals, an antenna connected? to one of said terminals and a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of a relatively short antenna connected to the other ofsaid terminals,

and means including a capacitor connected be tween said chassis and one of said terminals for applying to said terminal a component-of said noise voltage whereby said terminals are maintained at the same potential with respect to said voltage.

transformer primary winding provided with a pair of terminals, an antenna connected to one 5. In a radio apparatus including an input I minals are maintained at the same potential with 1 respect to said voltage.-

6. In a radio apparatus including an input transformer, the combination of a primary wind said lead of relatively low capacity value,con'

nected with the other ofv said terminals, and

ing for said transformer provided with a pair of means including a connection from the chassis to the antenna terminal for regulating and controlling the application of a component of a noise voltage to said terminals thereby to maintain said terminals at the same noise potential.

. VERNON D LANDON. 

